The present invention relates to a semiconductor device for controlling an ignition coil for an automobile or the like, and more particularly, to a semiconductor device comprising a transistor and a zener diode.
FIG. 4 shows an ignition circuit of an internal combustion engine of an automobile or the like. The ignition circuit comprises a semiconductor device 21 for controlling an ignition coil. An induction coil 22 and a power source 23 are connected to a cathode terminal (C) of the semiconductor device 21. A resistor 25 is connected to a gate terminal (G) of the semiconductor device 21. The gate terminal (G) receives a control signal 26 of the semiconductor device 21. An emitter terminal (E) of the semiconductor device 21 is grounded.
The semiconductor device 21 comprises an insulated gate bipolar transistor (hereinafter referred to as xe2x80x9cIGBTxe2x80x9d) 27. A zener diode 28 and a diode 29 are connected in series between a gate and a cathode of the IGBT 27.
In the ignition circuit, as the control signal 26 changes the IGBT 27 from the ON-state to the OFF-state, a predetermined voltage is developed on the primary side of the induction coil 22. Due to the zener diode 28, the voltage is always a constant voltage (hereinafter referred to as xe2x80x9cclamp voltagexe2x80x9d). As a result, a constant induced voltage is generated also on the secondary side of the induction coil 22, and this induced voltage is applied across an ignition plug 24. This sets sparks flying between the terminals, igniting an internal combustion engine or the like. In order to realize stable ignition, it is necessary to always apply a constant voltage across the ignition plug 24.
FIG. 5 is a cross sectional view of a conventional semiconductor device generally denoted at 150 for use in an ignition circuit. In the semiconductor device 150, a p-type well 2 is formed in an n-type silicon semiconductor substrate 1, and an n-type source region 3 is formed in the well 2. A gate electrode layer 9 is formed on the well 2 through an insulation layer 8. Meanwhile, an emitter electrode 10 electrically connected with the well 2 is formed on the well 2.
Further, a field oxide film 6 is formed on the semiconductor substrate 1, and a zener diode 7 of polycrystalline silicon is formed on the field oxide film 6. The zener diode 7 has a plurality of pn junction surfaces, and each pn junction surface is approximately perpendicular to a surface of the semiconductor substrate 1.
In FIG. 5, a diode (denoted at 29 in FIG. 4) is formed in series with the zener diode 7. In addition, the insulation layer 8 is formed on the zener diode 7, and a gate electrode 11 and a collector electrode 12 are electrically connected with the zener diode 7 through a hole formed in the insulation layer 8. The gate electrode 11 and the gate electrode layer 9 are electrically connected with each other.
Further, a p-type guard ring layer 4 is formed on the semiconductor substrate 1 below the zener diode 7.
A glass coat layer 33 of silicon nitride is formed as a protection film on a top surface of the semiconductor substrate 1.
On the other hand, an n-type buffer layer 14 and a p-type collector layer 15 are stacked one atop the other in this order on a back surface of the semiconductor substrate 1.
At last, a collector electrode 16 is formed on the p-type collector layer 15.
However, in an ignition circuit in which the semiconductor device 150 shown in FIG. 5 is used in the semiconductor device 21 shown in FIG. 4, if the semiconductor device 150 is switched (on/off) over a few million cycles, for example, the clamp voltage increases, and therefore, the voltage applied across the ignition plug 24 changes largely.
The present invention aims at providing a semiconductor device for preventing an increased clamp voltage and applying a constant voltage across an ignition plug in an ignition circuit.
As a result of intensive research, the inventor of the present invention discovered that while a semiconductor device is heated in the presence of hydrogen while forming a glass coat layer of the semiconductor device, hydrogen enters into the glass coat layer, and in turn the hydrogen diffuses in a zener diode during switching and a zener voltage accordingly changes.
In short, the present invention is directed to a semiconductor device comprising a transistor whose collector is connected to an induction coil and a zener diode connected between said collector and a gate of said transistor, wherein a constant induced voltage is generated in said induction coil in accordance with a signal supplied to said gate, said semiconductor device comprising: a semiconductor substrate; a transistor formed on said semiconductor substrate; a zener diode formed on said semiconductor substrate such that a pn junction surface is approximately perpendicular to a surface of said semiconductor substrate; and a glass coat layer formed on said semiconductor substrate so as to coat said zener diode, wherein said glass coat layer is an silicon oxide film.
Since the glass coat layer is made of silicon oxide, it is not necessary, unlike customarily practiced, to execute a step of forming the glass coat layer in a hydrogen-reduced atmosphere. This prevents diffusion of hydrogen in the zener diode during switching, and hence, to reduce a change in clamp voltage within the semiconductor device.
In addition, the present invention is directed to a semiconductor device comprising a transistor whose collector is connected to an induction coil and a zener diode connected between said collector and a gate of said transistor, wherein a constant induced voltage is generated in said induction coil in accordance with a signal supplied to said gate, said semiconductor device comprising: a semiconductor substrate; a transistor formed on said semiconductor substrate; a zener diode formed on said semiconductor substrate such that a pn junction surface is approximately perpendicular to a surface of said semiconductor substrate; and a glass coat layer of silicon nitride formed on said semiconductor substrate, wherein said glass coat layer is formed except for an area above said pn junction surface of said zener diode.
With this structure as well, it is possible to prevent diffusion of hydrogen in the zener diode and reduce a change in clamp voltage within the semiconductor device.
The glass coat layer of silicon nitride may be formed except for at least in an area above the pn junction surface of the zener diode, so that a change in clamp voltage within the semiconductor device is effectively suppressed.
The zener diode may include a plurality of pn junction surfaces.
It is preferable that an n-type region of said zener diode is connected to said gate of said transistor and a p-type region of said zener diode is connected to said collector of said transistor.
The zener diode is formed preferably on a field oxide film formed on said semiconductor substrate, and an insulation film is formed preferably between said zener diode and said glass coat layer.
The zener diode may be made of polycrystalline silicon.
Further, in the semiconductor device, a diode of a forward direction opposite to said zener diode may be disposed in series with said zener diode between said collector and said gate of said transistor.
The transistor is preferably an insulated gate bipolar transistor.
As described clearly above, application of the semiconductor device according to the present invention to an ignition circuit makes it possible to repeat stable ignition.